1. Field of the Invention
The present invention relates generally to a data transmission/reception apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a data transmission/reception apparatus and method for improving reliability of transmission data bits.
2. Description of the Related Art
In reality, in a communication system, it is impossible to receive a transmitted signal without any distortion or noise. Particularly, a mobile communication system that transmits and receives signals through a wireless network is more susceptible to the distortion or noise, compared with a communication system that transmits and receives signals through a wired network.
Therefore, various techniques for minimizing the influence of the distortion or noise have been proposed, and an error control coding technique is one of the typical proposed techniques. Codes used for the error control coding technique are classified into memoryless codes and memory codes. The memoryless codes include linear block codes, and the memory codes include convolutional codes and turbo codes. A device for creating such codes is called a “channel encoder,” and its outputs can be divided into systematic bits and parity bits according to the error control coding technique in use. The turbo codes are typically used for the error control coding technique for separately outputting the systematic bits and the parity bits. Of course, in addition to the turbo codes, systematic convolutional codes, a kind of the convolutional codes, are used to separately output the systematic bits and the parity bits. Here, the systematic bits mean actual signals to be transmitted, and the parity bits are signals added to correct a possible transmission error of the systematic bits during decoding. However, even in the case of the error control-coded signals, if a burst error occurs in the systematic bits or parity bits, it is not easy to correct the burst error. Such a phenomenon frequently occurs when a signal passes through a fading channel, and an interleaving technique is typically used to prevent this phenomenon.
The interleaving technique is used to more efficiently overcome the burst error by dispersing a defective part into several positions instead of concentrating the defective part on a particular position.
The interleaved signal undergoes symbol mapping in a digital modulator. Here, if an order of the modulator is increased, the number of bits included in one symbol is also increased. Particularly, in the case of a high-order modulation technique of over 16QAM (16-ary Quadrature Amplitude Modulation), one symbol includes 4 or more information bits, and the information bits can be classified according to their reliability. Here, as to the reliability, in a process of modulating one symbol by a transmitter, a symbol representing two bits in a macro region like the left/right quadrants or upper/lower quadrants on the X/Y-axis has “high reliability,” and a symbol representing two bits in a micro region has “low reliability.”
FIG. 1 schematically illustrates a structure of a transmitter in an HSDPA (High Speed Downlink Packet Access) mobile communication system. As illustrated, the transmitter includes a channel encoder, an interleaver and a modulator.
Referring to FIG. 1, input information bits to which CRC (Cyclic Redundancy Check) bits, or error detection data, are added in a CRC generator 110, are provided to a channel encoder 120, and the channel encoder 120 encodes the CRC bit-added input information bits through a predetermined coding process, and outputs coded bits, i.e., systematic bits S and parity bits P. The channel encoder 120 has at least one code rate in order to encode the information bits. The code rate may become ½ or ¾. In addition, when the channel encoder 120 supports a plurality of code rates through symbol puncturing or symbol repetition based on a rate R=⅓ or ⅕ mode code, an operation of selecting a particular code rate from the supportable code rates is required. In FIG. 1, for example, the channel encoder 120 determines a code rate under the control of a controller 160. The coded bits are subject to rate matching in a rate matcher 130. Commonly, the rate matching is performed through repetition and/or puncturing on the coded bits, when a transport channel is subject to multiplexing or the output symbols of the channel encoder 120 are not identical in number to the symbols transmitted over the air. The puncturing or repetition function of the rate matcher 130 is identical to the puncturing or repetition function performed to adjust a code rate of the channel encoder 120, the functions can be united. That is, the channel encoder 120 and the rate matcher 130 can be integrated into one block, but they are separately illustrated in FIG. 1, for the sake of convenience. The coded bits rate-matched by the rate matcher 130 are subject to interleaving in an interleaver 140. The interleaving operation is performed to minimize a data loss even though data is lost during transmission. The interleaved coded bits are subject to symbol mapping in a modulator 150 according to a modulation technique of QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation) or 64QAM. The controller 160 controls a coding operation of the channel encoder 120 and a modulation technique of the modulator 150 according to a current state of a radio channel. In the HSDPA mobile communication system, AMCS (Adaptive Modulation and Coding Scheme) is used for the controller 160 in order to adaptively select one of the modulation techniques QPSK, 8PSK, 16QAM and 64QAM according to the radio environment.
Though not illustrated in the drawing, a CDMA mobile communication system spreads transmission data with a Walsh code W and a PN (Pseudo Noise) orthogonal code (PN) so that a corresponding UE (User Equipment), or a mobile terminal, can identify a channel over which the data is transmitted, and a Node B, or a base station, which transmits the data.
In the transmitter structure stated above, as a matter of course, systematic bits and parity bits output from the channel encoder 120 have different priorities. In other words, in the case where errors occur in transmission data at a certain rate, the transmission data can be decoded more correctly at a receiver when the errors occur in the parity bits, compared with when the errors occur in the systematic bits. The reason is because, as stated above, the systematic bits are the actual data bits, while the parity bits are the bits added to correct the transmission errors during decoding. For this, a symbol mapping (SMP) technique has been proposed, and the SMP technique is disclosed in Korean patent application No. 2001-17925, filed by the applicant on Apr. 4, 2001 the contents of which are incorporated herein by reference.
The SMP technique is a technique for increasing system performance by reducing an error probability of the systematic bits having higher priority than the parity bits. That is, the SMP technique enables the modulator 150 to map the systematic bits with higher priority to the bits with higher reliability among the bits constituting a symbol, and map the parity bits with lower priority to the bits with lower reliability, during symbol mapping based on a predetermined modulation technique. Therefore, in the transmitter of the conventional mobile communication system, it is necessary to improve the interleaver 140 which interleaves coded bits regardless of their priority. That is, in order to apply the SMP technique, the interleaver 140 must be improved such that it can separately interleave the systematic bits and the parity bits.